1. Field of the Invention
This invention relates to a burn rate modifier for nitrocellulose base propellants. More particularly, a cellulose acetate deterrent is gradationally diffused into the propellant, with the maximum concentration of deterrent on the surface.
2. Description of Related Art
Smokeless propellant powder compositions containing a nitrocellulose base, either alone or in combination with other propellant bases, represent the most common gun powder in use today.
When the base is nitrocellulose, the propellant is referred to as a single base propellant. When the base is a mixture of nitrocellulose and nitroglycerin, the base is referred to as a double base propellant. When the base is a mixture of nitrocellulose, nitroglycerin and nitroguanidine, the propellant is referred to as a triple base propellant.
The propellant base is provided in either spherical (spheroidal) or modified spherical (oblate spheroids) globular powders manufactured by either a batch process or a continuous process. There are many different commercially available types of globular propellant powders, the difference primarily being in terms of the web (thickness), grain size, amount of nitroglycerin (an energy booster) and deterrent composition (to slow burning). The ammunition maker may also look to other powder types such as extruded or flake if globular powders do not give the proper ballistics.
The propellant powders have a high burn rate and may require the presence of a burn deterrent to reduce the initial burn rate of the powder composition and to impart a burn rate gradient to produce a high projectile velocity while preventing unduly high chamber pressures.
Among the deterrents known for nitrocellulose base propellants are linear polyesters as disclosed in U.S. Pat. No. 3,798,085 to Mellow and a polycaprolactone polymer as disclosed in U.S. Pat. No. 4,950,342 to Canterberry. Both the Mellow and the Canterberry patents are incorporated by reference in their entirety herein. Deterrents are classified as either "plasticizer type" or "barrier type". The plasticizer type deterrent diffuses into the propellent grains while the barrier type is normally not capable of diffusion into the propellent grains and coats the surface.
U.S. Pat. No. 4,354,884 to Williams discloses that single and double base propellants are usually coated from an aqueous solution containing dissolved deterrent. When the water is driven off, a coating of deterrent remains behind. With triple base propellants, nitroguanidine is water soluble and a nonaqueous solvent is required. One suitable solvent is methyl alcohol. The triple base propellant is immersed in the nonaqueous solution containing dissolved deterrent for a desired time, water rinsed and dried.
It is desirable to diffuse the deterrent into the propellant grains to establish a concentration gradient. The concentration of the deterrent is highest along the outside surface of the propellant grain and decreases to approximately zero at some point within the propellant grain. This concentration gradient slows down the burn rate when the propellant grains are large, reducing ballistic pressure. The burn rate increases as the size of the propellant decreases, maintaining a constant ballistic pressure.
If the concentration gradient changes, typically heat causes the deterrent to migrate inward, a negative ballistic effect occurs. The burn rate at the surface of the propellent grains increases, leading to increased ballistic pressure. As the grain size decreases, the increased amount of deterrent reduces the burn rate, leading to a drop in pressure.
Ballistic stability is the capability of the deterrent to remain in the original concentration gradient without migrating. A problem with currently used deterrents is ballistic instability. When exposed to elevated temperatures (65.degree. C. and higher) the prior art deterrents migrate, causing the ballistic performance to change.
There exists, therefore, a need for a deterrent for a nitrocellulose base propellant that has greater ballistic stability than those of the prior art.